CDS (I) 2021 General Knowledge · Question 108 of 120

Correct — D, Faraday's law of electromagnetic induction. Three of the four entries in this list are hand rules, and a hand rule exists to fix a direction in space. The right-hand thumb rule does it most directly: grip the wire with the right hand, thumb along the current, and the curled fingers give the direction of the magnetic field circling it. Fleming's two rules each set field, current and motion at right angles on three fingers, so a candidate who knows any two of the three can read off the third, including the field. Faraday's law is not of that kind at all. It is a quantitative statement — the emf induced in a circuit is proportional to the rate at which the magnetic flux through it changes — and it fixes how big the induced emf is, not which way anything points. The direction of the induced current is supplied by a separate statement, Lenz's law, which says the induced current opposes the change that produced it.

• (a)Right-hand thumb rule — This is the field-direction rule of the set. Thumb along the current, curled fingers along the magnetic field lines encircling the conductor — it does exactly what the stem asks about.
• (b)Fleming's left-hand rule — The motor rule. Forefinger for field, middle finger for current, thumb for the force, all mutually perpendicular — a geometric rule that ties a field direction to the other two.
• (c)Fleming's right-hand rule — The generator rule. Forefinger for field, thumb for the motion of the conductor, middle finger for the induced current — again a rule about directions in space.

Electromagnetism is taught through a division of labour. The quantitative laws say how much: Faraday's law gives the size of an induced emf from the rate of change of flux. The hand rules say which way: the right-hand thumb rule for the field around a current, Fleming's left hand for the force on a current in a field, Fleming's right hand for the current induced when a conductor moves in a field, and Lenz's law for the sense of that induced current.

Sorting the list into rules and laws answers the question before any physics is recalled. Three entries are named after hands and one is named a law, and the odd one out is the answer. Be exact about what the hand rules actually deliver, since the stem's wording is loose: strictly, the right-hand thumb rule is the only one whose output is the field, while Fleming's rules output a force and an induced current. They still let the field direction be recovered from the other two quantities, which is why the key rests on the sharper distinction — Faraday's law hands back a number and no direction at all. The companion fact to carry is that Lenz's law is the statement that supplies the missing sense, and that it is an expression of conservation of energy.

• The right-hand thumb rule gives the direction of the magnetic field around a straight current-carrying conductor.
• Fleming's left-hand rule gives the direction of the force on a current-carrying conductor in a magnetic field — the motor rule.
• Fleming's right-hand rule gives the direction of the current induced in a conductor moving in a magnetic field — the generator rule.
• Faraday's law states that the induced emf is proportional to the rate of change of magnetic flux through the circuit.
• The direction of an induced current comes from Lenz's law, which says it opposes the change producing it.

• Mixing up the two Fleming rules — left hand for the motor and the force, right hand for the generator and the induced current.
• Assuming Faraday's law also settles the direction of the induced current; that is Lenz's law.
• Reading the right-hand thumb rule as a rule about force rather than about the field.

As an odd-one-out among the rules of electromagnetism, or as a direct match of each rule to the quantity whose direction it gives.

• practice — not a real PYQ

  The direction of the current induced in a coil is given by

  • (a)Faraday's law
  • (b)Lenz's law
  • (c)Ohm's law
  • (d)Joule's law

  Answer(b) Lenz's law — the induced current always flows so as to oppose the change in flux that caused it, which is the direction rule that accompanies Faraday's quantitative law.

• practice — not a real PYQ

  Fleming's left-hand rule is used to find the direction of

  • (a)the induced current in a generator
  • (b)the magnetic field around a wire
  • (c)the force on a current-carrying conductor in a magnetic field
  • (d)the emf induced in a coil

  Answer(c) the force on a current-carrying conductor in a magnetic field — the motor rule, with forefinger for field, middle finger for current and thumb for force.